1. Field of the Invention
The present invention relates to an air-fuel ratio control apparatus of an internal combustion engine, and particularly to an apparatus having a mechanism for discharging adsorbed fuel vapor that carries out air-fuel ratio feedback control based on fuel vapor amount and fuel tank pressure.
2. Related Art
A conventional air-fuel ratio control apparatus is disclosed in, for example, Japanese Patent Application Laid-Open No. 8-14089. According to such an apparatus, an air-fuel ratio feedback correction amount is calculated, a stored air-fuel ratio learning value is read, and air-fuel ratio feedback control is carried out by using the air-fuel ratio feedback correction amount and the air-fuel ratio learning value. Further, fuel temperature in a fuel tank is detected. When the air-fuel ratio feedback control is carried out and a detected value of the fuel temperature is less than a predetermined temperature, the air-fuel ratio learning value is updated by using the air-fuel ratio feedback correction amount. Meanwhile, when air-fuel ratio feedback control is carried out and the detected value of the fuel temperature is equal to or higher than the temperature criterion value, updating of the air-fuel ratio learning value is prohibited. That is, when the detected value of the fuel temperature is higher than the temperature criterion value and a large amount of fuel vapor is generated in the fuel tank, fuel vapor introduced into an intake pipe is provided without being adsorbed to a canister, and erroneous learning operation by the fuel vapor is prevented.
However, generally, a relationship between fuel temperature and a fuel evaporation amount significantly differs depending on the kind of fuel. When the kind of fuel differs, volatility known by, for example, Reid vapor pressure RVP, differs. FIG. 15 is a diagram showing a relationship among fuel temperature (gasoline temperature), Reid vapor pressure and a fuel evaporation amount. As is evident from the diagram, as the gasoline temperature increases, or as the Reid vapor pressure increases, the fuel evaporation amount increases.
In this case, in the above-described conventional apparatus, when the temperature criterion value is set with fuel having high volatility (high RVP) as a reference, the temperature criterion value should be set to a small value (low temperature). However, as a result, updating of the air-fuel ratio learning value is normally prohibited. Further, when the temperature criterion value is set with fuel having low volatility (small RVP) as a reference, the temperature criterion value is set to a large value (high temperature), and erroneous learning is carried out.
Accordingly, it is an object of the present invention to provide an engine air-fuel ratio control apparatus capable of preventing erroneous learning while properly setting a learning frequency to thereby carry out highly accurate air-fuel ratio control.
The present invention is applied to an internal combustion engine having a fuel vapor discharging mechanism for temporarily adsorbing fuel vapor generated in a fuel tank in a canister and discharging the adsorbed fuel vapor to an engine intake system. In this environment, the present invention calculates a feedback correction amount based on an oxygen concentration in exhaust gas and executes air-fuel ratio feedback control by using the feedback correction amount.
More specifically, the present invention includes a controller that updates an air-fuel ratio learning value by using the feedback correction amount when the air-fuel ratio feedback control is being carried out. The controller also detects pressure in the fuel tank and prohibits updating of the air-fuel ratio learning value when the detected tank inner pressure exceeds a predetermined criterion value.
By prohibiting/permitting updating of the learning value based on tank inner pressure while monitoring an actual amount of fuel vapor fed to the side of the canister, the controller can properly update air-fuel ratio learning value in accordance with the amount of the fuel vapor. That is, even when various fuels having different volatilities are used, the determination can be properly carried out, even when a relationship between fuel temperature and a fuel evaporation amount significantly differs depending on the kind of fuel. As a result, erroneous learning is prevented, and highly accurate air-fuel ratio control can thus be carried out.